Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen
By Stephen Carpenter, Chair, Nina F. Caraco, David L. Correll, Robert W. Howarth, Andrew N. Sharpley, and Val H. Smith
Summary
Runoff from our farms and cities is a major source of phosphorus (P) and nitrogen (N) entering rivers, lakes, and coastal waters. Acid rain and airborne pollutants generated by human activities also supply N to surface waters. These nutrient sources are called "nonpoint" because they involve widely dispersed activities. Nonpoint inputs are difficult to measure and regulate because of their dispersed origins and because they vary with the seasons and the weather. Yet nonpoint inputs are the major source of water pollution in the United States today, and their impacts are profound. In aquatic ecosystems, over-enrichment with P and N causes a wide range of problems, including toxic algal blooms, loss of oxygen, fish kills, loss of seagrass beds and other aquatic vegetation, degradation of coral reefs, and loss of biodiversity - including species important to commercial and sport fisheries and shellfish industries. Thus, nutrient fouling seriously degrades our marine and freshwater resources and impairs their use for industry, agriculture, recreation, drinking water, and other purposes.
Based on our review of the scientific literature, we are certain that:
- Eutrophication caused by over-enrichment with P and N is a widespread problem in rivers, lakes, estuaries, and coastal oceans.
- Nonpoint pollution is a major source of P and N to surface waters of the United States. The major sources of nonpoint pollution are agriculture and urban activity, including industry and transportation.
- In the U.S. and many other nations, inputs of P and N to agriculture in the form of fertilizers exceed outputs of those nutrients in the form of crops.
- High densities of livestock have created situations in which manure production exceeds the needs of crops to which the manure is applied. The density of animals on the land is directly related to nutrient flows to aquatic ecosystems.
- Excess fertilization and manure production cause a P surplus, which accumulates in soil. Some of this surplus is transported in soil runoff to aquatic ecosystems.
- Excess fertilization and manure production create a N surplus on agricultural lands. Surplus N is mobile in many soils, and much leaches into surface waters or percolates into groundwater. Surplus N can also volatilize to the atmosphere and be redeposited far downwind as acid rain or dry pollutants that may eventually reach distant aquatic ecosystems.
If current practices continue, nonpoint pollution of surface waters is virtually certain to increase in the future. Such an outcome is not inevitable, however, because a number of technologies, land use practices, and conservation measures are available that can decrease the flow of nonpoint P and N into surface waters.
From our review of the available scientific information, we are confident that:
- Nonpoint pollution of surface waters with P and N could be decreased by reducing excess nutrient flows in agricultural systems, reducing farm and urban runoff, and reducing N emissions from fossil fuel burning.
- Eutrophication of aquatic ecosystems can be reversed by decreasing input rates of P and N. However, rates of recovery are highly variable, and recovery is often slow.
The panel finds that the roots of the problem of nonpoint pollution and eutrophication are well understood scientifically. There is a critical need for creative efforts to translate this understanding into effective policies and practices that will lead to protection and recovery of our aquatic resources.
